<template>
  <div v-if="prev || next" class="page-nav">
    <p class="inner">
      <span v-if="prev" class="prev">
        <a v-if="prev.type === 'external'" class="prev" :href="prev.path" target="_blank" rel="noopener noreferrer">
          {{ prev.title || prev.path }}

          <OutboundLink />
        </a>

        <RouterLink v-else class="prev" :to="prev.path">
          <a-icon type="left" />
          {{ prev.title || prev.path }}
        </RouterLink>
      </span>

      <span v-if="next" class="next">
        <a v-if="next.type === 'external'" :href="next.path" target="_blank" rel="noopener noreferrer">
          {{ next.title || next.path }}

          <OutboundLink />
        </a>

        <RouterLink v-else :to="next.path">
          {{ next.title || next.path }}
          <a-icon type="right" />
        </RouterLink>
      </span>
    </p>
  </div>
</template>

<script>
import { resolvePage } from '../util'
import isString from 'lodash/isString'
import isNil from 'lodash/isNil'

export default {
  name: 'PageNav',

  props: ['sidebarItems'],

  computed: {
    prev() {
      return resolvePageLink(LINK_TYPES.PREV, this)
    },

    next() {
      return resolvePageLink(LINK_TYPES.NEXT, this)
    }
  }
}

function resolvePrev(page, items) {
  return find(page, items, -1)
}

function resolveNext(page, items) {
  return find(page, items, 1)
}

const LINK_TYPES = {
  NEXT: {
    resolveLink: resolveNext,
    getThemeLinkConfig: ({ nextLinks }) => nextLinks,
    getPageLinkConfig: ({ frontmatter }) => frontmatter.next
  },
  PREV: {
    resolveLink: resolvePrev,
    getThemeLinkConfig: ({ prevLinks }) => prevLinks,
    getPageLinkConfig: ({ frontmatter }) => frontmatter.prev
  }
}

function resolvePageLink(linkType, { $themeConfig, $page, $route, $site, sidebarItems }) {
  const { resolveLink, getThemeLinkConfig, getPageLinkConfig } = linkType

  // Get link config from theme
  const themeLinkConfig = getThemeLinkConfig($themeConfig)

  // Get link config from current page
  const pageLinkConfig = getPageLinkConfig($page)

  // Page link config will overwrite global theme link config if defined
  const link = isNil(pageLinkConfig) ? themeLinkConfig : pageLinkConfig

  if (link === false) {
    return
  } else if (isString(link)) {
    return resolvePage($site.pages, link, $route.path)
  } else {
    return resolveLink($page, sidebarItems)
  }
}

function find(page, items, offset) {
  const res = []
  flatten(items, res)
  for (let i = 0; i < res.length; i++) {
    const cur = res[i]
    if (cur.type === 'page' && cur.path === decodeURIComponent(page.path)) {
      return res[i + offset]
    }
  }
}

function flatten(items, res) {
  for (let i = 0, l = items.length; i < l; i++) {
    if (items[i].type === 'group') {
      flatten(items[i].children || [], res)
    } else {
      res.push(items[i])
    }
  }
}
</script>

<style lang="less">
@import '../styles/palette.less';
@import '../styles/wrapper.less';
.page-nav {
  // @extend $wrapper;
  & {
    .wrapper;
  }
  
  padding-top: 1rem;
  padding-bottom: 0;

  .inner {
    min-height: 2rem;
    margin-top: 0;
    border-top: 1px solid @borderColor;
    padding-top: 1.75rem;
    overflow: auto; // clear float
  }

  .next {
    float: right;
  }
}
</style>
